“The price of success is hardworking dedicated to the job at hand, and the determination that whether we win or lose, we have applied the best of ourselves to the task at hand.”
I dedicate this thesis to my wife Dr. Eva Keji and my children.
This thesis represents not only my work at the keyboard, but my gratitude for many that made this work a success. First and foremost, I would like to express my profound and honest appreciation to my research supervisors Prof / Dr. Ahmed Ramadan Abdul-Aziz, Prof / Dr. Husham El Rifay and Dr. Mohammed El Habrouk University of Alexandria College of Electrical Engineering Department of Power and Machines, for guiding me during my course module epoch and also supervising my thesis to be realized. My special thanks goes to Dr. El Habrouk for rejecting my works in order to broaden my horizon on the development of power systems in general and specifically my topic of concern, without his support I would not realize what am missing and the entire effort wouldn’t be realized. Their vision has genuinely enthused me with practice to convey and present my thesis as clearly as possible. It was really a great honor to be a student under their supervision. I am extremely grateful for what they offered me during my course modules till my thesis supervision. I am also grateful to the Professors and Lectures who taught me; Engineers, teaching assistance, Staff of Alexandria University and not forgetting the people who supported me directly or indirectly to complete the research work.
Secondly to my parents and entire family who encouraged me to finish my MSc program despite the difficulties I went through during my stay abroad am really thankful to you. Special thanks and gratitude goes to my elder Sister Stella Lurit for her unwavering and sincere support despite of difficulties I can’t really express my joy for your support.
Last but not the least I would like to express my heartfelt thanks to my spouse Dr. Eva Keji for encouraging me to start my MSc in Alexandria University while endeavoring with the family and kids during my absence, encouraging me by providing me with conducive environment to concentrate in making this research a reality.
I Tim Tongun Emmanuel declare that no part of my work has been submitted in support of an application for degree or qualification from this or any other University or Institution. I confirm that proper credit has been given within this thesis where reference has been made to the work of others.
The thesis explains the different types of mitigation techniques of current harmonics used in power systems. It also surveys different types of active power filters control techniques that are used and reference current generation techniques used for active power filter and resorts to the best, which is the Double Capacitor Double Switch (DCDS) filter. This thesis also presents an online Artificial Neural Network (ANN) harmonic estimation technique suitable for generating the reference current estimation of Shunt Active Power Filter(SAPF). The control strategy used to control the filter is the hysteresis control. The Filter used in the proposed system is the DCDS, which is one of Switched Capacitor Active Power Filters (SCAPF). A generalized model of switching function block is developed first. The proposed DCDS circuit is derived from the generalized model. The DCDS capacitance is calculated using parallel resonance equation. The advantages of the filter are that it embraces cheaper elements than the standard inverter APFs, it is easy to control and its technology is feasible. The filter can inject the required harmonics into the supply. The circuit is simulated using MATLAB and Simulink. The switching frequency used is in the range of 2.5 kHz to 12.5 kHz and the largest capacitor utilized does not exceed 102.3µF. Moreover, the thesis compares between the FFT and ANN techniques, which shows the superiority of the ANN technique for reference current generation of the proposed active power filter (DCDS).
Over the past few years the sudden increase in the use of nonlinear loads such as personal computers and TV sets creates harmonic and Power Factor (pf) problems. Although such loads consume relatively small amount of power, the large number of these loads results in huge distortion in our power quality. One major element of the power quality is the harmonic (signal distortion). The main objective of the thesis is to investigate and propose the best technique used for mitigating current harmonics in power systems. This thesis surveys many harmonic mitigation techniques used and concludes to the proposed technique to be used in the future.
The proposed circuit consists of two semiconductor switches and two capacitors and an inductor. Such configuration forms the Switched Capacitor Active Power Filter (SCAPF) and is capable of current harmonic mitigation as well as generating variable reactive current. The SCAPF mitigates the THD to very low value compared to other techniques. Among the different plethora of SCAPF is the double capacitor double switch (DCDS) configuration.
The proposed Active filter is as from its name consists of two switches and two capacitors. The switches are used to pass fundamental and switching frequencies while the two capacitors are to attenuate the fundamental and pass the higher frequencies only. The switching pattern techniques are surveyed and the hysteresis current control (HCC) was found to be the most suitable technique to generate the switching signal.
The reference current generation of the signal was found using the Artificial Neural Network (ANN) technique, the radial basis function (RBF) technique is used to generate the negative harmonics.
The modeling of the thesis was implemented using MATLAB/Simulink Package-Sim Power System toolbox.
In simulation the DCDS filter was simulated with three different types of loads: inductive, capacitor and mixed load. The DCDS was compared with the voltage source inverter (VSI) active power filter using the three different loadings. The DCDS shows a promising result better than the VSI active power filter for single phase applications.
LIST OF SYMBOLS
Dc link Capacitor
Current in time domain
Root mean square current
Short circuit current
Number of samples per cycle
Number from 1,2,3……
Power of fundamental
Power of capacitor
Power of inductor
Power of dc component
Reactive power of fundamental
Reactive power of harmonic
Voltage in time domain
Voltage of fundamental
Voltage of capacitor
Voltage of dc link
Dc link voltage in time domain
Voltage of harmonic
Load voltage in time domain
Root mean square voltage
Neural network weight
Neural network input
Neural network output
Neural network hidden layer elements
LIST OF GREEK LETTERS
?h Center of radial basis function
Neural network learning rate
Spread of the radial basis function
Switching angular frequency
Phase angle between voltage and current
Phase angle between harmonic voltage and current
LIST OF ABREVIATION
Ant colony optimization
Artificial neural network
Active power filter
Adjustable speed drive
Current source inverter
Double capacitor double switch
Discrete Fourier transform
Delta modulation control
Digital signal processing
Fast Fourier transform
Fuzzy logic controller
Hysteresis current control
High pass filter
High voltage direct current
Institute of electrical and electronic engineers
Insulated gate bipolar junction transistors
Kilo volt ampere
Kilo volt ampere reactive
Low pass filter
Mega volt ampere
One cycle control
Pulse amplitude modulation
Point of common coupling
Passive power filter
Particle swarm optimization
Pulse width modulation
Radial basis function
Radial basis function neural network
Root mean square root
Shunt active power filter
Switch capacitor active power filter
Switched capacitor circuit
Switched mode power system
Synchronous reference frame
Variable speed constant frequency
Variable speed drive
LIST OF FIGURES
2 Fundamental and harmonics Figure 1.1
3 Structure of nonlinear loads Figure 1.2
4 AC to DC nonlinear loads Figure 1.3
5 Current fed nonlinear load Figure 1.4
5 Voltage fed nonlinear load Figure 1.5
6 Mixed nonlinear load Figure 1.6
11 Reactance mitigation techniques Figure 2.1
12 Tuned harmonic mitigation techniques Figure 2.2
14 Generalized blocks diagram of active power filter (APF) Figure 2.3
18 Open loop control technique Figure 2.4
19 Vdc control technique Figure 2.5
19 Hysteresis current control (HCC) technique Figure 2.6
20 Linear current control technique Figure 2.7
20 One cycle control (OCC) technique Figure 2.8
21 Space vector control (SVC) technique Figure 2.9
21 Delta modulation control (DMC) technique Figure 2.10
23 Particle swarm optimization Figure 2.11
23 Ant colony optimization Figure 2.12
23 Vdc estimation using Artificial neural network (ANN) Figure 2.13
24 Vdc control using fuzzy logic controller (FLC) Figure 2.14
24 Vdc control using Neuro-fuzzy controller (NFC) Figure 2.15
25 Voltage source inverter (VSI) active power filter (APF) Figure 2.16
26 Cascaded multilevel active power filter Figure 2.17
26 Diode clamp multilevel active power filter Figure 2.18
27 Flying capacitor multilevel active power filter Figure 2.19
27 Current source inverter (CSI) active power filter Figure 2.20
28 Lattice structure (LS) active power filter Figure 2.21
29 Voltage regulator (VR) active power filter Figure 2.22
29 Switched capacitor active power filter (SCAPF) Figure 2.23
31 Series active power filter Figure 2.24
31 Unified power quality conditioners (UPQC) Figure 2.25
32 Shunt active power filter and parallel passive power filter Figure 2.26
32 Series active power filter and parallel passive power filter Figure 2.27
32 Parallel passive power filter with series active power filter Figure 2.28
35 High pass filter (HPF) Figure 3.1
36 Low pass filter (LPF) Figure 3.2
36 Instantaneous reactive power (IRP) algorithm Figure 3.3
37 Synchronous reference frame (SRF) algorithm Figure 3.4
39 Ant colony optimization (ACO) Figure 3.5
40 Biological neuron Figure 3.6
41 Artificial neural network for reference current estimation Figure 3.7
42 Types of transfer functions Figure 3.8
44 Multilayer artificial neural network Figure 3.9
45 Radial basis function neural network Figure 3.10
48 Generalized block diagram of SCAPF Figure 4.1
51 Mathematical model of switched capacitor active power filter (SCAPF) Figure 4.2
52 Phase relationships between voltage and current in active power filter Figure 4.3
53 Proposed filter circuit Figure 4.4
54 Power and power factor in linear and nonlinear loads Figure 4.5
56 Hysteresis current control (HCC) Figure 4.6
59 Flowchart of reference current generation Figure 4.7
61 Simulink block diagram of the proposed circuit with nonlinear loads Figure 4.8
64 Inductive Source voltage waveform without filter Figure 4.9
66 Inductive Source voltage FFT without filter Figure 4.10
67 Inductive load Source current waveform without filter Figure 4.11
67 Inductive load Source current FFT without filter Figure 4.12
67 Capacitive Load Source current waveform without filter Figure 4.13
68 Capacitive Source current FFT without filter Figure 4.14
68 Mixed Load Source current waveform without filter Figure 4.15
68 Mixed Source current FFT without filter Figure 4.16
69 Inductive Source voltage waveform using VSI filter Figure 4.17
69 Inductive Source current waveform (a) using VSI filter Figure 4.18
70 Inductive Source current waveform (b) using VSI filter Figure 4.19
70 Inductive Source current waveform (c) using VSI filter Figure 4.20
70 Magnified Max Inductive Source current FFT using VSI filter Figure 4.21
71 VSI Filter current waveform (a) for Inductive load Figure 4.22
71 VSI Filter current waveform (b) for Inductive load Figure 4.23
71 VSI Filter current waveform (c) for Inductive load Figure 4.24
72 Capacitive Source voltage waveform using VSI filter Figure 4.25
72 Capacitive Source current waveform (a) using VSI filter Figure 4.26
72 Capacitive Source current waveform (b) using VSI filter Figure 4.27
73 Capacitive Source current waveform (c) using VSI filter Figure 4.28
73 Magnified Max capacitive Source current FFT using VSI filter Figure 4.29
73 VSI Filter current waveform (b) for capacitive load Figure 4.30
74 VSI Filter current waveform (b) for capacitive load Figure 4.31
74 VSI Filter current waveform (c) for capacitive load Figure 4.32
74 Mixed Source voltage waveform using VSI filter Figure 4.33
75 Mixed Source current waveform (a) using VSI filter Figure 4.34
75 Mixed Source current waveform (b) using VSI filter Figure 4.35
75 Mixed Source current waveform (c) using VSI filter Figure 4.36
76 Magnified Max Mixed Source current FFT using VSI filter Figure 4.37
76 VSI Filter current waveform (a) for mixed load Figure 4.38
76 VSI Filter current waveform (b) for mixed load Figure 4.39
77 VSI Filter current waveform (c) for mixed load Figure 4.40
77 Inductive Source Voltage waveform using DCDS filter Figure 4.41
77 Inductive Source current waveform (a) using DCDS filter Figure 4.42
78 Inductive Source current waveform (b) using DCDS filter Figure 4.43
78 Inductive Source current waveform (c) using DCDS filter Figure 4.44
78 Magnified Max Inductive Source current FFT using DCDS filter Figure 4.45
79 DCDS Filter current waveform (a) for Inductive load Figure 4.46
79 DCDS Filter current waveform (b) for Inductive load Figure 4.47
79 DCDS Filter current waveform (c) for Inductive load Figure 4.48
80 Capacitive Source voltage waveform using DCDS filter Figure 4.49
80 Capacitive Source current waveform (a) using DCDS filter Figure 4.50
80 Capacitive Source current waveform (b) using DCDS filter Figure 4.51
81 Capacitive Source current waveform (c) using DCDS filter Figure 4.52
81 Magnified Max capacitive Source current FFT using DCDS filter Figure 4.53
81 DCDS Filter current waveform (b) for capacitive load Figure 4.54
82 DCDS Filter current waveform (b) for capacitive load Figure 4.55
82 DCDS Filter current waveform (c) for capacitive load Figure 4.56
83 Mixed Source voltage waveform using DCDS filter Figure 4.57
83 Mixed Source current waveform (a) using DCDS filter Figure 4.58
83 Mixed Source current waveform (b) using DCDS filter Figure 4.59
84 Mixed Source current waveform (c) using DCDS filter Figure 4.60
84 Magnified Max Mixed Source current FFT using DCDS filter Figure 4.61
84 DCDS Filter current waveform (a) for mixed load Figure 4.62
85 DCDS Filter current waveform (b) for mixed load Figure 4.63
85 DCDS Filter current waveform (c) for mixed load Figure 4.64
85 Reference current and actual load current versus time(s) Figure 4.65
86 Reference current minus actual load current versus time(s) Figure 4.66
86 PWM versus Time(s) Figure 4.67
LIST OF TABLES
10 Harmonic current distortion limits (IEEE Standard 512) Table 2.1
10 Harmonic voltage distortion limits (IEEE Standard 512) Table 2.2
12 Advantages and disadvantages of PPF Table 2.3
17 Classification APF based on compensated parameters Table 2.4
30 Comparison of switched capacitor APF Table 2.5
33 General comparison of different types of APF Table 2.6
63 Filter calculation results Table 4.1
64 Simulation parameters Table 4.2
65 Comparison between ANN technique for VSI and DCDS filter Table 4.3
List of symbols vi
List of greek letters viii
List of abreviations ix
List of figures xi
List of tables v
Table of contents xvii
Chapter 1: Power quality overview 1
1.1 Introduction 1
1.2 Causes of power quality problems 1
1.3 Harmonics in power systems 1
1.3.1 Causes of harmonics in power systems 2
1.4 Classification of nonlinear loads 2
1.4.1 Non-solid-state nonlinear loads 3
184.108.40.206 Electrical machines 3
1.4.2 Solid-state nonlinear loads 3
220.127.116.11 Converter based nonlinear loads 3
18.104.22.168 Nature based nonlinear loads 4
22.214.171.124 Supply system based classification 6
1.5 Objectives 7
1.6 Thesis layout 7
Chapter 2: Literature review 8
2.1 Introduction 8
2.2 Power quality issues 8
2.3 Fundamentals of harmonic formulae 9
2.3.1 The effects of harmonics in power systems 9
2.3.2 Harmonic standards 10
2.4 Harmonic mitigation techniques 10
2.4.1 Passive power filter technique 11
126.96.36.199 Reactance technique. 11
188.8.131.52 Tuned harmonic filter 11
2.4.2 Active power filter technique 11
184.108.40.206 Description of standard APF. 11
2.5 Classification of active power filters 13
2.5.1 Power rating and speed of response. 15
220.127.116.11 Low power APF 15
18.104.22.168 Medium power APF 16
22.214.171.124 High power APF 16
2.5.2 Classification based on compensated parameters 16
2.5.3 Classification based on control techniques 18
126.96.36.199 Open loop control systems 18
188.8.131.52 Closed loop control systems 18
184.108.40.206 Optimization technique 22
220.127.116.11 Other techniques 23
2.5.4 Classification based on topology. 24
18.104.22.168 Shunt APF. 25
22.214.171.124 Series APF 30
126.96.36.199 Other combination 31
2.6 Summary 34
Chapter 3: Reference current generation 35
3.1 Introduction 35
3.1.1 Reference signal synthesis 35
188.8.131.52 High pass filter . 35
184.108.40.206 Low pass filter. 35
3.1.2 Reference signal estimation 36
220.127.116.11 Time domain technique. 36
18.104.22.168 Frequency domain technique 38
22.214.171.124 Other techniques 38
3.2 Background of neural network 39
3.2.1 Structure of neurons 40
126.96.36.199 Basic components of biological neurons 40
3.3 Artificial neural network 41
3.3.1 Weighting factors 41
3.3.2 Summation function 41
3.3.3 Activation function 42
3.3.4 Scaling and limiting 42
3.3.5 Output function 42
3.3.6 Error function and back-propagated value 43
3.3.7 Learning function 43
3.4 Types of artificial neural network 43
3.4.1 Multilayer perceptron’s. 44
3.4.2 Radial basis function network. 44
3.4.3 Why Use ANN? 45
188.8.131.52 Other advantages of ANN 45
3.5 Summary 46
Chapter 4: Proposed Circuit 47
4.1 Introduction 47
4.2 General topology of switched capacitor APF 47
4.2.1 Energy storage elements in SCAPF filter. 49
184.108.40.206 Role of energy storage elements SCAPF filter. 49
4.3 General mathematical model for SCAPF 50
4.3.1 Switching function analysis of SCAPF 50
4.3.2 Proposed circuit topology (DCDS) 53
4.3.3 Operation and performance of DCDS filter 53
220.127.116.11 Mode of operation of DCDS filter 54
4.4 Power in system loads 55
4.4.1 Power factor in linear loads 56
4.4.2 Power factor in harmonics loads 56
4.5 Proposed control technique 58
4.5.1 Hysteresis current control 58
4.5.2 Proposed reference current generation technique 59
4.6.1 Radial basis function 60
18.104.22.168 Concept of the RBF 60
4.7 Effect of resonance on DCDS filter 62
4.7.1 Quality factor and LCR tuned circuits 62
4.8 Filter design 62
4.9 Simulation results and discussions 63
4.9.1 Comparison between ANN technique for VSI and DCDS filter 63
4.9.2 Simulation scenarios 63
4.9.3 Simulation of nonlinear loads without filter connection 66
22.214.171.124 Inductive loads waveforms and FFT 67
126.96.36.199 Capacitive loads waveforms and FFT 67
188.8.131.52 Mixed loads waveforms and FFT 68
4.9.4 Simulation of filters with nonlinear loads. 69
184.108.40.206 Simulation of VSI filter with inductive loads 69
220.127.116.11 Simulationof VSI filter with capacitive loads 72
18.104.22.168 Simulation of VSI filter with mixed loads. 74
22.214.171.124 Simulation of DCDS filter with inductive loads. 78
126.96.36.199 Simulation of DCDS filter with capacitive loads 79
188.8.131.52 Simulation of DCDS filter with mixed loads 79
4.10 Summary 63
Chapter 5: Conclusion and future work 87
5.1 Conclusion 87
5.2 Contribution of thesis 87
5.3 Future work 88
Appendix A. A
Appendix B. B
Appendix C. C
POWER QUALITY OVERVIEW
In preceding decades, electricity has primarily been used to power electric motors, incandescent lighting and resistive heating. Nowadays with the increasing types of power generations technique, electricity is used mostly by electronic loads: consumer electronics such as televisions, industrial electronics such as ASD, and high efficiency lighting. The widespread use of electronic equipment; led to a complete change of electric loads nature 1-3. The availability of distributed energy resources has led to the development and complexity of loads used in power systems today. The inception of power quality in the other hand contributes to the clean use and uninterrupted power. If the two major factors are provided by the utility, the consumers’ expectations are met. The utility needs the consumers to use clean power so that not to pollute the power grid. The use of power electronics devices saves the consumers from fuel issues, but in the other hand creates harmonics currents which tend to be a menace to the power systems. These current harmonics are the main factor which results to voltage harmonics. If the voltage is being polluted by the current harmonics it means that often consumers using the same voltage will be affected; which is against the utility policy. Therefore, by mitigating the targeted current harmonics results also in mitigating the voltage harmonics indirectly 4. The issue of polluted power has drawn the interest of multiple researchers on different ways to mitigate the harmonic problems over the years and immense papers have been published in the scientific journals and conferences. Researchers are struggling to devise a new strategy on almost harmonic free systems and to solve the problem 5-6. The mitigation technique is used near the harmonic generator before reaching the power system 7.
CAUSES OF POWER QUALITY PROBLEMS
In the past, most of the loads were linear loads. A linear load is the load where the voltage across it and the current through follow the same pattern. For example, if the voltage is sinusoidal the current is also sinusoidal (harmonic free). Calculations of power, and pf in such loads were straight forward and most of these loads were fed from sinusoidal voltage and the power factor in these loads is simply Cos? the angle between the voltage and current 8. However, over the last 50 years, large nonlinear loads such static power converters used in HVDC transmission started to appear and due to the semiconductor devices used in such converters, the linear relationship between the voltage and current is not valid any more 9-10. Such large nonlinear loads introduced current harmonics to the power system and resulting creating voltage harmonic. When the voltage is being affected means the entire system is polluted because they are sharing the same voltage source 11.
HARMONICS IN POWER SYSTEM
Harmonics is one of the categories and characteristics of Electromagnetic phenomena in power system which affects power quality 12; it falls under waveform distortion with typical spectral content of 0-100th and typical voltage magnitude of 0-20%. These harmonics causes heating in conductors and torque pulsating in electrical motors. The core point of this thesis is the harmonic reduction.
Causes of Harmonics in Power Systems
In free harmonic system the current is sinusoidally they are usually 50 or 60 Hz. In linear loads the current drawn is in phase with the voltage while the nonlinear loads the current is out of phase with the voltage. When a rectifier(nonlinear) is connected to a system it draws a non-sinusoidal current, the current waveform tends to be out of phase with the voltage. Fourier series is the best technique used to analyze the harmonic series 13. The voltage harmonics are mostly produced when harmonic currents dominate the power systems leading to voltage deterioration due to the source impedance. The higher the source impedance the higher the voltage distortion and vice versa. The loads that commonly distort the power systems are: computers, printers’ Fluorescent lamps battery chargers and ASD 14. Figure 1.1 shows an example an example of nonlinear load and its waveform.
CLASSIFICATION OF NONLINEAR LOADS
The nonlinear loads can generally be classified based into two major groups, solid state nonlinear load and non-solid, Figure 1.2 shows the entire classification of nonlinear load used in power systems 15.
Non-Solid-State Nonlinear Loads
This type of nonlinear loads as the name infers shows that there is no solid-state materials or power electronics converters used, its example is electrical machines.
When connected to supply they behave like nonlinear loads due saturation in the magnetic material of the machines and electromagnetic devices. There are many reasons letting the machines to behave as nonlinear load e.g. effect of teeth and slots in rotating machines, transformers in no load or light loads and magnetic ballast all produced harmonics which pollutes the system 15.
Solid-State Nonlinear Loads
The solid state nonlinear loads circuit consists of solid state materials and power electronic converters; they draw non-sinusoidal current from the supply. This non-sinusoidal current consists of reactive, harmonic and fundamental current and the active power components of the current. They use various AC-DC converters, AC voltage controllers, cyclo-converters, or a mixer of them in front-end converter 15-16. In the single-phase configuration, the harmonic currents and the reactive power are drawn from the AC mains. The solid state nonlinear loads can be classified based on three categories: –
Converter based Nonlinear Load
Nature based Nonlinear Load
Supply based Nonlinear Load
Converter based Nonlinear Load
The converter-based uses power converters for converting AC to DC power for utilization; they are mostly AC voltage controllers, cyclo-converter or combination of both. These types can be classified into two as shown in Figure 1.3: –
AC-DC Converter Based.
AC Controller Based.
AC-DC Converter Based.
It is the mostly used converter in power systems starting from few watts to a Megawatt rating. They are mostly found in microwave ovens, SMPS, computers, fax machines, battery chargers, HVDC transmission systems, electric traction, ASD and the list continues 16. They exhibit poor pf at the AC mains generally due to harmonics only, but with reactive power as well, the example is shown in Figure 1.3.
AC Controller Based
Other harmonic generators use the main source for controlling their AC RMS voltage to control the voltage across the electrical load to control the physical process. Some of these loads include speed controllers, soft starters, heating controllers, AC voltage regulator in fans, lighting controllers. The best example of this load is given below.
A cyclo-converter is a frequency changer that converts ac power at one input frequency to output power at a different frequency with one-stage conversion process and vice versa 15- 16. They draw harmonic currents not only at higher order harmonics but also at sub-harmonics and reactive power and exhibit a very poor power factor at the AC mains. Some of the examples of such nonlinear loads include cyclo-converter fed large-rating synchronous motor drives in cement mills, ore crushing plants, large-rating squirrel cage induction motors, slip energy recovery scheme of wound rotor induction motor drives, VSCF (variable speed constant frequency) generating systems, and so on.
Nature Based Nonlinear Load
Most of the nonlinear loads behave as either stiff current fed type or as stiff voltage fed type, or a mixture of the two. The stiff current fed loads normally consist of AC-DC converters with constant DC current load and a predetermined harmonic pattern in the AC mains with reactive power burden. The voltage stiff loads consist of generally AC-DC converters with a large DC capacitor at the DC bus to provide ideal DC voltage source for the remaining process of solid-state conversion. Since the analysis of the behavior and remedy for mitigation of power quality problems of these types of loads depend reasonably on this classification, it becomes relevant and important to select a proper compensator 16.
Current Fed Nonlinear load.
Voltage fed Nonlinear Load.
Mixed Nonlinear Load (mixture of the two).
Current Fed Nonlinear load
The stiff current fed types of nonlinear loads generally have predetermined pattern of harmonics and sometimes they have reactive power burden on the AC mains. They have flat current waveform drawn from the AC mains with a low value of crest factor 15. They typically consist of AC-DC converters feeding DC motor drives, magnet power supplies, field excitation system of the alternators, controlled AC-DC converters used to derive DC current source for feeding current source inverter supplying large-rating AC motor drives, HVDC transmission systems, and so on. Figure 1.4 shows such current fed type of nonlinear load.
Voltage Fed Nonlinear Load
The stiff voltage types of nonlinear loads behave as sink of harmonic currents. Typical example of such load is an AC-DC converter with a large DC capacitor at its DC bus. The idea is to provide an ideal DC voltage source for the remaining process of solid-state conversion and it draws peaky current from the AC mains with high crest factor (as shown in Figure 1.5). They generally do not have reactive power requirement, but they have much greater amount of harmonic currents drawn from the AC mains. Examples of such loads include SMPS, battery chargers, front-end converters of voltage source inverter fed AC motor drives, electronic ballasts, and most of the electronic appliances 16.
Mixed Nonlinear Load
The mixed nonlinear loads are combination of current fed and voltage fed types of loads. A group of nonlinear loads and a combination of linear and nonlinear loads fall under this category. Most of the electrical loads consisting of solid-state converters behave as these types of nonlinear loads, shown in Figure 1.6.
Supply System-Based Classification
This section deals with the type of supply feeding the nonlinear loads such as: single phase three phase three wire (ADS) and three phases’ four wire supplies. There are many nonlinear single-phase loads distributed on a four-wire, three-phase supply system, such as computers, commercial lighting, and so on.
Two-Wire Nonlinear Loads
There are a very large number of single-phase nonlinear loads supplied by the two-wire single-phase AC mains. All these loads consisting of single-phase diode rectifiers, semi-converters, and thyristor converters behave as nonlinear loads 15. They draw harmonic currents and sometimes also the reactive power from the AC mains. Typical examples of such loads are power supplies, electronic fan regulators, electronic ballasts, computers, television sets, and traction this are typical example of voltage fed type nonlinear load.
Three-phase three-wire nonlinear loads
These loads inject harmonic currents, they draw reactive power from the AC mains and sometimes they also have unbalanced currents. These nonlinear loads are in large numbers and consume major amount of electric power. Typical examples are ASDs using DC and AC motors 16-17, HVDC transmission systems, and wind power conversion. These are typical example current fed type nonlinear load.
Three-Phase Four-wire Nonlinear Loads
Numerous single-phase nonlinear loads may be supplied from the three-phase AC mains with the neutral conductor. Apart from harmonic currents, reactive power, and unbalanced currents, they also cause excessive neutral current due to harmonic currents and unbalancing of these loads on three phases 17. Typical examples are computer loads and electronic ballasts-based vapor lighting systems. Besides, they cause voltage distortion and voltage imbalance at the PCC and some potential at the neutral terminal, its examples are current fed type nonlinear load.
The main aim of this research work is to design a SCAPF to mitigate the current harmonics injected by nonlinear load. To achieve this main aim, the following objectives were set: –
Fully investigate the causes of current harmonics, comparing different types of APFs used for current harmonic mitigation, speed, elements used, reference current generation, control techniques and cost.
Derive mathematical formulae for the proposed SCAPF circuit.
Investigate the best reference current generation and the control techniques used to generate the switching pattern for the proposed filter.
Simulate the proposed SCAPF circuit using Matlab/Simulink software.
This thesis investigates and proposes the best technique to be used for mitigating current harmonics in power systems. The proposed technique found was the SCAPF; the SCAPF can be connected in shunt for ‘retrofit application’. The advantages of this filter (SCAPF) is to generate negative harmonics to cancel the imposed harmonics by the nonlinear load and to improve the pf of the system.
This thesis is divided into five chapters. The first chapter is an introduction to the problems affecting power quality; the chapter explains different types of nonlinear loads connected to power systems and brief description of harmonics in power systems.
The second chapter deals with, the causes of harmonics and the standards used to measure THD, the different types of harmonic mitigation techniques used in the power industry, the review on classifications of general APFs used nowadays and comparing them according to the classifications to reached the best filter to be used in the future.
The third chapter deals with the review of general reference current generation used for APF, and zeroing in the best reference current technique which suits the proposed APF. In this chapter, the best technique used for generating the reference current technique is found to be the ANN and among the different types of ANN the function fitting (RBFNN) is found to be the best candidate that suits the thesis due to its accuracy and fast performance than the multilayer perceptron’s.
The fourth chapter emphasizes an on on-line control of the proposed circuit for the SCAPF, the mode of operation of the proposed DCDS active power filer is shown, the reference current generation technique is discussed with the control technique to get the suitable switching pattern for the proposed SCAPF. The proposed circuit is tested for mitigation of the current harmonics and comparing it with VSI shunt APF using the same nonlinear loads. Simulation results are compared in a comparison table.
Chapter five contains the conclusions and the future work of this thesis.
Since the sudden increase in numbers of small nonlinear loads, such as computers, T.V sets, etc., the issue of power quality became vital to the power electronics industry. It is highly noticed that the impending supply quality legislation will soon demand the reduction of harmonic and reactive power levels in the networks. The solution for many installations will be, is to install global power system conditioners, (which are active filters as the scope of this chapter is concern) at the PCC 1. Hence the study of active power system conditioner solutions is becoming extremely popular. The term active power filter (APF) is a broad term which is applied to a group of power electronic circuits incorporating power switching devices and passive energy storage circuit elements, such as inductors and capacitors. The functions of these circuits vary depending on the applications. The APFs mostly used to mitigate the current harmonics in supply networks in the low and medium voltage distribution. These functions may be combined in a single circuit or in separate active filters. In the previous years, there has been a quantum surveys on active filters. Several publications have described the development of active filtering techniques 1-12; some of them focus mainly on circuit configurations and their possible interconnections. Others review the control techniques associated with some of these circuits. However, there is a lack of published material which provides an overview such that designers and users of active filters can evaluate various techniques in a subjective fashion. This contribution is aimed at filling the different types of active power filters and the suitable control technique used for each active filter. Before delving to active power filter, a brief explanation about power quality problems is shown; why active filter is needed, what are the problems it’s capable of solving and why is it better than the rest of the mitigation techniques.
2.2. POWER QUALITY ISSUES
The subject of power quality is very broad by nature. It covers all aspects of power system engineering, from generation (production), transmission and distribution level to end-user (customers). Since power quality is a vast topic the scope of this research is going to focused on some harmonic mitigation techniques only. To mitigate harmonics in power systems; utilities, end users, architects, and civil engineers as well as manufacturers must be involved. These professionals must work together in developing solutions to power quality problems 17: –
Electric utility managers and designers must build and operate systems that consider the interaction between customer facilities and power system.
Electric producers should make sure the source voltage used by consumers is clean.
Consumers must use less harmonic loads not to pollute their neighbor’s voltage.
The Architects and civil engineers must design buildings complying with power quality mitigations.
Manufacturers and equipment engineers must design devices that are compatible with the power system. This might mean a lower level of harmonic generation or less sensitivity to voltage distortions.
Engineers must be able to devise ride through capabilities of distributed generators (e.g., wind and solar generating plants).
2.3. FUNDAMENTALS OF HARMONICS FORMULAE
Harmonics provides a mathematical analysis of distortions to a current and voltage waveform, the distorted current and voltage can be found by Fourier series. In harmonic studies, the term THD is used to measure the harmonic voltage and current levels 17. It is shown below: –
?THD?_I=(?_(h=2)^??I_h^2 )/I_1 2.1
?THD?_V=(?_(h=2)^??V_h^2 )/V_1 2.2
Where I_n and V_n and are the RMS values of the nth harmonic of the currents and voltages, and I_1 and V_1 are the RMS of fundamental current and voltage. Normally the maximum harmonic number, n, in both distortion functions is limited to between the 19th and the 50th harmonics, depending on the desired accuracy 17. When deal with nonlinear loads the power factor will be known as the displacement factor since nonlinear loads has distorted current 18. The periodic non-sinusoidal waveforms can be expressed in terms of Fourier series. Each in the series known as component of the distorted waveform and harmonic frequency is the integer multiples of the fundamental frequency. The harmonics can be even and odd but even doesn’t exist in power systems, if they exist means imperfect gating’s of the electronic switches 15 17. Another important harmonic is the triplex harmonics which exist in unbalanced three phase load. The third harmonic neutral currents are the product of the third harmonic phase current and the three. Transformer winding connections have a significant impact on the flow of tripled harmonic currents caused by three-phase nonlinear loads 17. For the grounded wye-delta transformer, the tripled harmonic currents enter the wye side and since they are in phase, they add in the neutral. The delta winding connection tends to trap the harmonic current since no path of exit is provided for the current for this reason delta star transformers are mostly used in power stations and substations.
2.3.1. The Effects of Harmonics in Power Systems
Harmonic currents and voltages results in several power quality problems for power systems. They are as follows: –
Decrease the total pf and therefore increase the volt-ampere ratings of the power system equipment.
Voltage distorted at the point of common coupling (PCC) because of the voltage drop in the transformer and line impedance.
Failure of pf correction capacitor due to resonant condition that can occur at a harmonic associated with the nonlinear load.
Reduction in the efficiency of the rotating machines due to increasing iron and copper losses.
Metering and instrumentation error particularly when resonant conditions exist.
Overheating of transformers resulting in insulation damage and failure.
Interference with communication networks.
2.3.2. Harmonic Standards
These are standards that set the maximum allowable levels of the current and voltage distortion that apply to the individual consumers of electrical energy 15 10. The two Tables show the IEEE Standard 512-1992, it specifies recommended limits on both current and voltage distortions are shown in Table 2.1 and 2.2.
Table 2.1: Harmonic current distortion limits (IEEE Standard 512)
Maximum Harmonic current distortion at PCC (% of fundamental)
Harmonic order (Odd harmonics)
ISC/ IL h